Nixzmary Brown’s mother yesterday admitted she stood idle as her sadistic husband viciously beat her daughter because she feared for herself – and, besides, the child was insolent and sneaky.

“Nixzmary had an attitude,” Nixzaliz Santiago, 27, told The Post in a jailhouse interview at Rikers Island.

“She is isolated because she creates mischief in the middle of the night,” she said, still speaking of her dead daughter in the present tense.

A whimpering Santiago said she cried to her evil hubby, “Leave her alone!” at about 11 p.m. Tuesday, as cops say the maniacal fiend was launching his merciless attack on the hollow-eyed 7-year-old.

But it was the only intervention she offered.

“I was afraid of him,” said Santiago, wearing a gray prison suit, her bleached-blond hair worn loose down her back.

“If I were to interfere with the discipline of the children, he would have hit me.”

Even as the child was slowly dying from her beating – having been dunked in a tubful of ice-cold water and slammed around the bathroom – the mother tried to ignore the little girl’s cries for help.

Santiago said she dried off her daughter, put her in pajamas and laid her down on mattress in a backroom near a heater. She said that Nixzmary felt cold but that she thought that was from the water.

Then she left to join her husband.

“Mommy, don’t leave me,” the girl moaned. “Mommy, don’t leave me.”

Those were Nixzmary’s final words.

Only after the girl was dead and Santiago and Cesar Rodriguez, 27, were in a squad car did the mother finally confront the man who allegedly beat her daughter to death.

“Why did you do this to my baby?” she asked. “What was going through your head?”

“I didn’t know what I was doing,” Rodriguez said, according to Santiago. “I was blinded by rage, and I’m sorry.”

Santiago – eyes red, fingernails gnawed – methodically recounted the exchange at Rikers, where she is being held on manslaughter charges.

Though her husband is accused of committing the actual murder in the Brooklyn apartment, Santiago’s most critical comments weren’t about him.

They were about her daughter.

When she wasn’t bothering her siblings during her late-night shenanigans, sometimes hitting them with a wooden stick, Santiago said, Nixzmary would raid the refrigerator, drinking the baby’s expensive Enfamil and eating baby food.

Although school officials who first alerted authorities about possible abuse said the girl was malnourished, Santiago said that she fed her daughter but that sometimes Nixzmary refused to eat.

On several occasions, Santiago said, Nixzmary hid her food in a napkin and tossed it out the window.

One time, the mother said, instead of eating a slice of pizza, she stuffed it in the toilet.

Though Santiago witnessed Rodriguez’s severe punishment of the girl for months, she said she never considered his discipline to be abusive.

In fact, the wound that triggered one of the calls to the city’s child-welfare agency – a bruise and a cut over Nixzmary’s eye – did not involve Rodriguez at all, she said. Santiago said she was responsible.

Santiago said she pushed her daughter down after the girl ignored her plea to tell her about an injury to one of the other children. Nixzmary hit a wall and bruised her eye. Santiago said she hugged the girl and apologized. As for the bedroom’s being a torture chamber where, according to police, Nixzmary was tied to a chair, Santiago said that wasn’t true. She said the chair was there to hold up a table.

“I told the detectives that he never tied her up to the chair,” she said. “He never tortured her, and he never tied the chair to the radiator. That’s a lie.”

Santiago said the beginning of the end for Nixzmary came at 11 p.m. Tuesday, when the girl’s siblings fingered her for taking a container of yogurt out of the refrigerator, angering her stepfather.

Later, Rodriguez was having problems with his computer.

“Who was messing with my printer?” Rodriguez bellowed.

The children again fingered Nixzmary.

Santiago said she watched and wailed as Rodriguez lifted the skin-and-bones Nixzmary off the floor and carried her kicking and screaming through their Bedford-Stuyvesant apartment to the bathroom, where he flung her into the tub like a bathmat.

Rodriguez then drenched the fully clothed girl in cold water as she cowered in the tub, knees against her chest, her two hands on the sides of her head.

Santiago said she did nothing more to intervene for fear he would lash out at her.

She said she didn’t see what happened next. She followed the cries of a younger child into another room.

But when she heard a loud thump in the bathroom, she sprinted back to the bathtub, where Rodriguez was holding her semiconscious daughter.

After she returned, Rodriguez handed over her dripping-wet daughter and went to bed.

Although Nixzmary was nearly lifeless and moaning, another five hours would pass before outside help was summoned.

Throughout the night, Santiago said, she checked on the girl, poking her head in the door, asking, “Baby, are you OK?”

But when her oldest son came in at 4 a.m. and asked, “Mommy, why is Nixzmary not making any noise?” she knew something was wrong.

Santiago said she felt Nixzmary’s face and it was cold.

“Go check on my baby,” she said to Rodriguez. “I know something is happening to my baby.”

“Calm down, calm down,” Rodriguez replied. “She’s fine.”

After a while, he relented, saw his stepdaughter was not breathing and performed CPR.

But, even then, he was rough. Santiago thinks Rodriguez may have broken one of the girl’s ribs while pushing on her chest.

Police said Santiago ran upstairs to a neighbor’s apartment to get help. From there, she dialed 911.

Nixzmary was pronounced dead at the scene.

Santiago said that she had intervened once before when Rodriguez was hitting one of the kids and that he’d turned his rage on her. “He pushed me against the wall and grabbed me by my throat,” Santiago said. “I could barely breathe.”